The present invention is directed to doped zinc oxide compositions and methods for preparing such compositions.
Zinc oxide is used as a paint pigment and as an additive in the manufacture of plastics. Since zinc oxide is used primarily as a white pigment, any substantial coloration, e.g. yellow, is normally undesirable and is to be avoided. Moreover, optical properties such as reflectivity, index of refraction and resistance to discoloration by radiation have particular importance in specific applications.
Even substantially pure zinc oxide has been known to demonstrate a noticeable yellow color. While the precise reason for this yellow coloration cannot be stated with certainty, there have been published theories which might explain the optical mechanism involved.
Research published in 1968 (W. E. Vehse, W. A. Sibley, F. J. Keller, and Y. Chen, Phys. Rev., 167 [3] 828 (1968)) showed the presence of a 410 nm optical absorption band in zinc oxide and how this band was influenced by the incorporation of impurities and by irradiation of the zinc oxide using gamma and electrons. Research published in 1972 (D. R. Locker and J. M. Meese, IEEE Trans. Nuc. Sci., NS-19, 237 (1972)) ascribed the structural identity of the 410 nm band to the possibility of an F+center (an oxygen vacancy with one trapped electron). Research published in 1988 (J. C. Simpson and J. F. Cordaro, J. Appl. Phys., 63 [5] 1781 (1988)) showed the presence of a 0.3 eV deep level in the band gap of zinc oxide and ascribed this deep level to the F+ center of Locker and Meese. Simpson and Cordaro also suggested that the optical transition associated with the 410 nm band may be a charge transfer reaction from the valence band to the defect level.
This theory is based upon the premise that the optical absorption mechanism which is responsible for giving some zinc oxide pigments a yellow coloration is related to an F+center, or F+-like defect in the ZnO. Since the 410 nm absorption "band" is more like an absorption edge, the mechanism could likely involve the transfer of an electron from an extended state (band) to a localized state, rather than between localized states. The above premise does not, however, rule out that some species other than an F+center, such as a zinc interstitial, could be involved.
The electronic transition from an extended state to a localized state may be expressed by the following equation: ##STR1##
Consistent with the above theory, but not necessarily dependent upon it, it is an objective of the present invention to achieve zinc oxide compositions having enhanced optical properties. In particular, it is an objective of the present invention to employ dopants that will suppress the mechanism by which yellowing occurs, and thereby to obtain a whiter pigment. It is also an objective that the doped pigment have enhanced reflectivity, index of refraction and resistance to discoloration on exposure to radiation.